This application relates generally to systems and methods for image acquisition and, more specifically, to systems and methods for acquiring and reconstructing multi-slice computed tomography data to produce high signal-to-noise ratio images with minimal metal-induced artifacts.
Computerized tomography (CT) involves the imaging of the internal structure of an object by collecting several projection images in a single scan or several scans, and is widely used in the medical field to view the internal structure of selected portions of the human body. Typically, several two-dimensional projections are made of the object, and a three-dimensional representation of the object is constructed from the projections using various tomographic reconstruction methods. In the case of electronic portal imaging, megavolt therapeutic X-rays can be used to generate images. However, this method results in images of low contrast and quality, in addition to incidentally damaging healthy tissue. As a result, imaging with megavoltage (MV) radiation is used primarily for portal verification, that is, to confirm that the treatment volume is being radiated.
In some cases, the optimal x-ray beam energy for maximizing signal-to-noise ratios and minimizing doses for in vivo computed tomography scans is in the range of 70-130 kV. This X-ray photon energy range is optimal for resolving soft tissue contrast in the CT images. However, at these beam energies, high density or highly attenuating objects such as gold fillings or metal prostheses can cause streaks and other image artifacts to be present in the projection images and in the final reconstructions.
Metal-induced artifacts may be compensated either by refinements to the CT system using anti-scatter grids, or by software corrections using iterative or interpolation techniques. However, software correction is problematic since it is computationally intensive and requires long reconstruction time, which may result in longer per-patient imaging processing time. Furthermore, software correction may be unavailable when imaging highly dense metal objects such as gold dental work or hip joint prostheses. In such cases, these highly dense metal objects completely absorb the primary x-ray beam thereby prevent absorption coefficient data from being obtained.
Sometimes, computed tomography systems may operate at much higher beam energies (e.g., up to 7 MV) than conventional diagnostic CT systems. Because the higher energy x-rays can penetrate metal objects, these higher energy CT systems produce images that are devoid of metal artifacts. However, as a trade off, patient radiation doses received from MV CT scans are often higher than those from a conventional kV CT scan. Moreover, the MV images exhibit significantly degraded soft-tissue contrast and spatial resolutions compared to kV images. The degradation in soft tissue contrast resolution at high beam energies is the inherent loss of differential contrast due to reduced photoelectric absorption and reduced Compton scattering cross-sections. Another reason for the degradation in soft tissue contrast resolution at high beam energies is reduced detective quantum efficiencies (DQE) due to the high penetration of the x-ray beam through the conventional detector conversion layers designed for kV x-rays. In some cases, the conventional detector's DQE may be as low as 3% under high beam energy, and thus the conventional detector is undesirable for imaging soft tissue at high beam energy. To effectively image soft tissues using high energy, high DQE detectors are required for high energy CT scans. However, high DQE detectors require very thick and expensive conversion layers to absorb the primary beam. Thus, building a large-area detector for high energy CT which can be used to scan a volume efficiently (e.g., in a single rotation of the source-detector pair) can be prohibitively expensive. On the other hand, imaging a target volume using only a high DQE detector with a small area is not desirable, because it would require many rotations to cover the axial extent of the volume of interest.